An example of the shovel that includes the electrical energy storage device is a hybrid shovel. In general, in the hybrid shovel, a hydraulic pump is driven with an output of an engine to drive a hydraulic load, and an assist motor is driven with electric power supplied from the electrical energy storage device to assist the engine.
The electrical energy storage device includes an electrical energy storage part (an electrical energy storage unit or a storage battery) for storing electric power to supply the electric power if necessary. For example, an electrical double layer condenser or a lithium capacitor, or the like may be used as an electrical energy storage part.
An internal resistance of the capacitor depends on a temperature thereof such that the internal resistance becomes greater as the temperature becomes lower. A voltage variation when the capacitor is charged and discharged depends on the internal resistance of the capacitor such that the voltage variation becomes greater as the internal resistance becomes greater. Specifically, a capacitor voltage for a certain discharge current at a low temperature of the capacitor is lower than that at an ordinary temperature.
For example, a case is assumed where a shovel, in which the electrical energy storage device that has a capacitor with ordinary temperature requirements is installed, is operated in cold climates in which an outdoor temperature is minus 20 degree Celsius. In this case, the temperature of the capacitor at the time of starting the operation of the shovel is as low as the outdoor temperature, and thus the internal resistance of the capacitor is very great. If ordinary charge and discharge control is performed to generate a charge or discharge current of the capacitor, the voltage of the capacitor changes greatly. For example, when the voltage of the capacitor becomes extremely low, degradation of the capacitor is significantly accelerated.
In such a context, a hybrid type working machine that can restrain the charge or discharge current of the capacitor when the temperature of the capacitor becomes lower than or equal to a predetermined threshold is proposed (see Japanese Unexamined Patent Publication No. 2010-178446, for example, which is referred to as “Patent Document 1” hereinafter).
In general, the capacitor that is used for the electrical energy storage device of the shovel has such a characteristic that the electrical energy storage amount becomes smaller due to the degradation after the capacitor has been used for a long time. The electrical energy storage units other than the capacitor and the storage battery such a lithium ion battery also have the same degradation characteristic.
In general, it is known that a degradation manner (degradation speed) of the capacitor depends on the temperature and the voltage of the capacitor. The higher the temperature of the capacitor becomes, the higher the degradation speed becomes and thus the capacitor is degraded earlier. Further, the higher the voltage of the capacitor becomes, the higher the degradation speed becomes and thus the capacitor is degraded earlier. When the capacitor is degraded, the internal resistance becomes greater.
For example, there is a case where the internal resistance of the electrical double layer condenser at minus 40 degree Celsius is about 2.5 times higher than that at 20 degree Celsius. For example, there is a case where the internal resistance of the lithium capacitor at minus 20 degree Celsius is about 4.5 times higher than that at 20 degree Celsius.
Thus, if the shovel is operated at the site where the outdoor temperature is low, the internal resistance of the capacitor, which has already become high due to the degradation to a certain degree, becomes further high due to the low temperature, which causes the capacitor voltage to greatly change.
FIG. 1 is a diagram for illustrating a voltage variation of the capacitor at the low temperature. In FIG. 1, a solid line indicates the voltage variation of the capacitor where the outdoor temperature is ordinary (20 degree Celsius, for example) and the capacitor has an ordinary temperature, and a dotted line indicates the voltage variation of the capacitor where the outdoor temperature is low (minus 20 degree Celsius, for example) and the capacitor has a low temperature.
In FIG. 1, the discharge is started at time t1 and the discharge current Id1 is generated until time t2. After that, the charge is started at time t3 and the charge current Ic1 is generated until time t4. After that, the charge ends and the discharge starts at time t4, and the discharge current Id2 is generated until time t5.
In the case where the outdoor temperature is ordinary (20 degree Celsius, for example), when the discharge is started at time t1, the capacitor voltage slightly decreases due to a voltage drop that corresponds to the product of the discharge current and the internal resistance (see “A” portion in FIG. 1), as indicated by the solid line. At that time, because the temperature is ordinary and thus the internal resistance of the capacitor is not significantly great, the magnitude of the voltage drop is small. In this example, although the capacitor voltage further decreases due to the discharge from time t1 to time t2, it does not reach a system lower limit voltage. The system lower limit voltage is a lower limit value for the capacitor voltage that is set in a control system for the shovel. The charge and discharge control is performed by the control system such that the capacitor voltage does not fall below the system lower limit voltage.
When the discharge is stopped and the discharge current Id1 becomes 0 at time t2, the capacitor voltage slightly increases by an amount corresponding to the internal resistance (see “B” portion in FIG. 1). After that, when the charge is started at time t3, the capacitor voltage slightly increases due to a voltage rise that corresponds to the product of the charge current and the internal resistance (see “0” portion in FIG. 1), as indicated by the solid line. At that time, because the temperature is ordinary and thus the internal resistance of the capacitor is not significantly great, the magnitude of the voltage rise is small. Although the capacitor voltage further increases due to the charge from time t3 to time t4, it does not reach a system upper limit voltage. The system upper limit voltage is an upper limit value for the capacitor voltage that is set in a control system for the shovel. The charge and discharge control is performed by the control system such that the capacitor voltage does not exceed the system upper limit voltage.
When the charge ends and the discharge starts at time t4, the capacitor voltage decreases due to a voltage drop that corresponds to the product of the charge current, the discharge current and the internal resistance (see “D” portion in FIG. 1). At that time, because the temperature is ordinary and thus the internal resistance of the capacitor is not significantly great, the magnitude of the voltage drop is small. Although the capacitor voltage further decreases due to the discharge from time t4 to time t5, it does not reach the system lower limit voltage.
That is the variation in the capacitor voltage in the normal status indicated by the solid line in FIG. 1.
Next, the variation in the capacitor voltage in the case where the capacitor is at the low temperature is explained.
In the case where the outdoor temperature is low (minus 20 degree Celsius, for example), when the discharge is started at time t1, the capacitor voltage decreases due to a voltage fall that corresponds to the product of the discharge current and the internal resistance (see “E” portion in FIG. 1), as indicated by the dotted line. At that time, because the internal resistance of the capacitor is great due to the low temperature, the magnitude of the voltage fall is great and thus, in this example, the capacitor voltage drops below the system low limit voltage. If a system is configured such that the discharge limitation works when the capacitor voltage drops below the system low limit voltage, the ordinary discharge current Id1 cannot be generated due to the discharge limitation. As a result of this, the electronic control of the shovel is not appropriately performed, which can lead to a problem in the operations of the shovel.
In contrast, a case where the discharge limitation does not work when the capacitor voltage drops below the system low limit voltage is assumed. In this case, because the discharge current Id1 is generated from time t1 to time t2, the capacitor voltage, which has fallen below the system low limit voltage, continues to further decrease due to the discharge even through it decreases gradually (see “F” portion in FIG. 1). The lower limit voltage is a rated voltage that indicates a lower limit for use of the capacitor. For example, in the case of the lithium capacitor, when the capacitor voltage becomes smaller than or equal to the lower limit voltage, the degradation of the capacitor is significantly accelerated. In the case of the electrical double layer condenser, the lower limit voltage is 0, and thus the lower limit voltage is not set.
When the discharge is stopped and the discharge current Id1 becomes 0 at time t2, the capacitor voltage increases by an amount corresponding to the internal resistance. After that, when the charge is started at time t3, the capacitor voltage increases due to a voltage rise that corresponds to the product of the charge current and the internal resistance (see “G” portion in FIG. 1), as indicated by the dotted line. At that time, because the internal resistance of the capacitor is great due to the low temperature, the magnitude of the voltage rise is great and thus the capacitor voltage exceeds the system upper limit voltage. When the capacitor voltage exceeds the system upper limit voltage, the charge limitation works and the ordinary discharge current Ic1 cannot be generated. As a result of this, the electronic control of the shovel is not appropriately performed, which can lead to a problem in the operations of the shovel.
In contrast, a case where the charge limitation does not work when the capacitor voltage exceeds the system upper limit voltage is assumed. In this case, because the charge current Ic1 is generated from time t1 to time t2, the capacitor voltage, which has exceeded the system upper limit voltage, continues to further increase due to the charge even through it increases gradually (see “H” portion in FIG. 1). The upper limit voltage is a rated voltage that indicates an upper limit for use of the capacitor. For example, when the capacitor voltage becomes greater than or equal to the upper limit voltage, the degradation of the capacitor is significantly accelerated.
When the charge ends and the discharge starts at time t4, the capacitor voltage decreases due to a voltage drop that corresponds to the product of the charge current, the discharge current and the internal resistance. At that time, because the internal resistance of the capacitor is great due to the low temperature, the magnitude of the voltage fall is great and thus the capacitor voltage drops below the system low limit voltage again. If the discharge is continued, the capacitor voltage further decreases to be below the lower limit voltage.
In this way, when the internal resistance of the capacitor becomes great due to the low temperature, the voltage variation at the time of the charge and the discharge of the capacitor becomes great, which often leads to a situation where the system upper limit voltage or the system lower limit voltage is exceeded so that the charge and discharge control cannot be performed and the degradation of the capacitor is significantly accelerated.
In such a configuration in which the charge and discharge control is performed when the capacitor voltage is lower than a threshold, as disclosed in Patent Document 1, it is necessary to appropriately set the threshold. If the threshold for the system lower limit is too high, the discharge limit may easily work, because the capacitor voltage becomes lower than the threshold only if the temperature of the capacitor decreases by a small amount (i.e., only if the internal resistance of the capacitor increases by a small amount), which may lead to the problem in the operations of the shovel. On the other hand, if the threshold for the system lower limit is too low, the capacitor voltage does not easily becomes lower than the threshold even if the temperature of the capacitor becomes significantly low (i.e., even if the internal resistance of the capacitor becomes significantly great), which may accelerate the degradation of the capacitor due to the significant low capacitor voltage.
Here, if the internal resistance of the capacitor is constant at the ordinary temperature and the internal resistance of the capacitor always changes in the same way with the change in the temperature, it is possible to set the constant threshold. However, the internal resistance of the capacitor depends on the degradation level of the capacitor, and a rate of change in the internal resistance with respect to the change in the temperature depends on the degradation level of the capacitor. Specifically, in the case of the capacitor which has been degraded significantly, the internal resistance of the capacitor is great even at the ordinary temperature, and the change amount of the capacitor voltage at the time of the charge and the discharge is great. Thus, unless the threshold is set in consideration of the degradation level of the capacitor, the charge and discharge control cannot be performed appropriately as the degradation level of the capacitor becomes high.